Frequency modulator for digital transmissions

ABSTRACT

The invention relates to a method of transmitting digital data exhibiting a rate T by means of a frequency modulator able to modulate as a function of the data, a central carrier frequency f 0  at a first frequency value f 0 +¼T and/or a second frequency value f 0 −¼T. It comprises the step consisting in modulating the carrier frequency from one of the frequency values to the other during a time interval T, via successive frequency stages.

The invention relates to a method of transmitting digital data by meansof a frequency modulator based on a frequency modulation of minimumphase gradient or “Minimum Shift Keying” (MSK) type.

The subject of the invention is also the corresponding device.

The invention applies to all digital data transmissions using MSK-typefrequency modulation.

It applies in particular to the accurate determination of the positionof a mobile on the basis of necessary data transmitted between areference station and the mobile, both receiving satellite-basedpositioning signals.

For the determination of the absolute position of a mobile, use iscommonly made of satellite-based position measurement means, using forexample the radio signals emitted by the satellites of the GPS (GlobalPositioning System) or of other similar systems (GLONASS system, futureGALILEO system). The accuracy obtained goes from a few meters to a fewtens of meters.

In the GPS system, the signal emitted by a satellite is coded and thetime taken by the signal to reach the point to be located is used todetermine the distance between this satellite and this point, preferablycalled the pseudo-distance so as to take account of synchronizationerrors between the clock of the satellite and that of the station. Thesesynchronization errors are conventionally eliminated by calculation whenthe signals are received from at least four different satellites. Thedetermination of the distance between the point to be located andseveral satellites makes it possible, knowing the geographicalcoordinates of the satellites, to calculate the coordinates of the pointto be located, usually coordinates expressed as latitude, longitude andaltitude in a fixed terrestrial reference frame.

To determine the precise position of a mobile (accuracy of from acentimeter to a meter, as the case may be), a so-called “differentialGPS” procedure is used, which consists in using, at the level of themobile, for the calculation of its position, the errors noted withregard to each pseudo-distance at the level of a so-called referencestation of known position.

This procedure makes it possible to correct the position calculationerrors due in particular to trajectory deformations and to propagation.These errors are corrected, for example, by comparing for the referencestation its known position and its calculated position arising from themeasurement of the propagation time between the satellite and thereference station.

The digital data corresponding to these errors are transmitted by thereference station to the mobile. Traditionally, these digital data aretransmitted by radio using MSK-type frequency modulation, well-suited toan information bit rate that may be as much as 200 baud (bits persecond).

For greater information bit rates, such as 400 baud, the MSK-typefrequency modulation is no longer suitable since it exhibits too wide afrequency spectrum that decreases too slowly.

It is in fact recalled that the narrower the spectra of the referencestations and the faster they decrease, the more they can be juxtaposedon the same frequency band without them encroaching on one another andthus the more reference stations there can be.

Now, it is noted that curve a), representing in FIG. 1 the frequencyspectrum of an MSK modulation at 400 baud, exhibits at 1000 Hz asidelobe situated at around −38 dB from the central lobe, whereas onewants it to be situated at around −50 dB.

One solution consists in preceding the MSK frequency modulation by aGaussian low-pass filtering. One is then dealing with a Gaussian minimumphase gradient, or “GMSK” (Gaussian minimum shift keying) modulation,whose frequency spectrum is represented by curve b) of FIG. 1. Thespectral occupancy is better adapted than in the case of “MSK”modulation, but the GMSK modulation introduces undesirable inter-symbolcrosstalk.

It is recalled that inter-symbol crosstalk consists in the reception ofthe data (also termed symbols) being scrambled through the simultaneousreception of the correct data and of a tailoff of the previous data itemor even of the previous but one.

An important aim of the invention is therefore to propose a method and adevice exhibiting spectral occupancy equivalent to that exhibited byGMSK modulation, without introducing inter-symbol crosstalk.

Another aim of the invention is to propose a method that is easy toimplement.

To achieve these aims, the invention proposes a method of transmittingdigital data exhibiting a rate T by means of a frequency modulator ableto modulate as a function of the data, a central carrier frequency f0 ata first frequency value f0+¼T and/or a second frequency value f0−¼T,this process being characterized in that it comprises the stepconsisting in modulating the central carrier frequency f0 from one ofthe frequency values to the other during a time interval T, viasuccessive frequency stages.

Thus, instead of going directly from a first frequency valuecorresponding to a first value of a data item, to a second frequencyvalue corresponding to a second value of a data item, one goes from thefirst frequency to the second through successive frequency stages.

The frequency spectrum corresponding to these staged changes offrequency exhibits a faster decrease than the spectrum of an “MSK”modulation.

This staged change of frequency furthermore exhibits the advantage ofconsuming less energy at high frequency than during instantaneouschanges of frequency such as in the case of “MSK” modulation, thisenergy gain then being in part carried over to the central lobe of thespectrum, thus giving it a slightly wider useful band than in the caseof “MSK” modulation and resulting in a gain in the signal/noise ratiowith respect to “MSK”, for identical conditions.

According to a characteristic of the invention, the frequency stages forgoing from f0−¼T to f0+¼T are the same in absolute value as thefrequency stages for going from f0+¼T to f0−¼T.

The number of frequency stages for going from one of the frequencyvalues to the other is preferably equal to 16.

The method applies in particular when the digital data are transmittedbetween a reference station and a mobile of a satellite-basedpositioning system.

Also, the subject of the invention is not only the method oftransmitting data, the gist of which has just be described, but also adevice able to implement a frequency modulation at a rate T according totwo predetermined frequencies. This device comprises means for shapingthe said modulation as several frequency stages during a time intervalT.

This device comprises a microprocessor able to program the frequencystages and preferably, linked to the microprocessor, a device able toshape the modulation as a function of data to be transmitted and of theprogrammed stages.

Finally, a subject of the invention is a frequency modulator,characterized in that it comprises a device able to shape a frequencymodulation as described and a device for generating instantaneousfrequencies.

Other characteristics and advantages of the invention will becomeapparent on reading the detailed description which follows and which isgiven with reference to the appended drawings in which:

the curves represented diagrammatically in FIG. 1 illustrate thevariation of the frequency spectrum of a modulator of “MSK” type (curvea) and “GMSK” type (curve b);

FIGS. 2 a), 2 b) and 2 c) diagrammatically represent, as a function oftime t, respectively an example of binary data to be transmitted, andthe corresponding frequency and phase modulations;

FIG. 3 diagrammatically represents as a function of time t, thefrequency modulations corresponding to an input signal comprising thedata −1, 1, −1 for a conventional “MSK” modulation (curve a) and for anexemplary modulation according to the invention designated “MSK16”(curve b);

FIG. 4 diagrammatically represents as a function of time t, the “MSK”modulation (curve a) and “MSK16” modulation (curve b) for an inputsignal comprising the data −1, 1, 1, −1, 1, 1, −1, 1, −1, 1;

FIG. 5 diagrammatically represents as a function of time, the phasevariations corresponding to the said “MSK” modulation (curve a) and“MSK16” modulation (curve b) for an input signal comprising the data −1,1, 1, −1, 1, 1, −1, 1, −1, 1;

FIG. 6 diagrammatically represents the frequency spectra of an “MSK”modulation at 400 baud (curve a) and of an “MSK16” modulation at 400baud (curve b);

FIG. 7 diagrammatically represents an exemplary device able to implementthe method according to the invention.

The frequency modulation according to the invention is based on an“MSK”-type modulation.

The features of the frequency modulation of “MSK” type are brieflyrecalled on the basis of an example described in conjunction with FIGS.2 a), 2 b) and 2 c).

This is of course a constant-amplitude modulation.

The binary data to be transmitted, represented in FIG. 2 a), each have aduration T also referred to as the data rate. As represented in FIG. 2b), each data item is transmitted as a frequency, for a duration Taccording to the following characteristics:

a) +1 is represented by the first frequency f0+¼T,

b) −1 is represented by the second frequency f0−¼T,

f0 being the central carrier frequency.

With this frequency modulation may be associated over the duration T, avariation of the phase according to the following formula:$\begin{matrix}{\varphi = {\int\limits_{0}^{T}{2\pi\quad{f(t)}{\mathbb{d}t}}}} & (1)\end{matrix}$

The deviation ¼T is chosen in such a way that the corresponding phase(φ, represented in FIG. 2 c), varies linearly between −π/2 and π/2.

On reception, phase demodulation is often preferred to frequencydemodulation.

The method according to the invention consists in going from onefrequency to the other according to successive stages in such a way asto attain the desired frequency (f0±¼T) at the end of the duration T.Thus, at the end of a time T, the corresponding phase φ attains the samevalue (±π/2) as in the case of “MSK” modulation; this makes it possibleon reception to phase-demodulate the data received whether they havebeen modulated according to conventional “MSK” modulation or accordingto the invention.

In what follows, consideration is given to an example of frequencymodulation with 16 frequency stages, the carrier frequency f0 takingvalues between 1.6 and 3.5 MHz, in particular 1.8146 MHz, the rate Tcorresponding to 400 Hz and ¼T being equal to 100 Hz. The data thusmodulated in this HF (high frequency) range are transmitted by means ofa suitable antenna, in this instance a large antenna.

The number of stages may take other values such as, for example, 8 or32.

Illustrated in FIG. 3 is this example of frequency modulation for aninput signal comprising the data −1, 1, −1. Curve a) corresponds toconventional “MSK” modulation and exhibits two frequency states; curveb) corresponds to the modulation according to the invention referred toas “MSK16” since the modulation is shaped as 16 frequency stages.

As regards the “MSK16” curve (b), the frequency f0+¼T normalized in thefigure to +1 is attained at the 16th stage; likewise, the frequencyf0−¼T normalized in the figure to −1 is attained at the 16th stage. Inorder for a receiver able to demodulate data modulated according to an“MSK” modulation to be compatible with a modulation according to theinvention, “MSK16” for example, it is necessary for the phase to beequal to ±π/2 at the end of the time T. This is why, having regard torelation (1), certain frequency stages exceed (in absolute value) thefrequency to be attained.

The “MSK16” curve (b) which, in order to go from the frequency −1 to thefrequency +1 with a phase variation equal to ±π/2, exhibits 16 frequencystages of the form k_(i)×(¼T), i varying from 1 to 16, was obtained withthe following coefficients k_(i): −0.703 −0.413 −0.12 +0.173 +0.466+0.76 +1.053 +1.346 +1.64 +1.933 +2.226 +2.053 +1.791 +1.528 +1.266 +1

Likewise, the 16 frequency stages for going from +1 to −1 were obtainedwith the opposite coefficients: +0.703 +0.413 +0.12 −0.173 −0.466 −0.76−1.053 −1.346 −1.64 −1.933 −2.226 −2.053 −1.791 −1.528 −1.266 −1

Of course, as in the case of “MSK”, the frequency remains stable whenthe data do not change; illustrated in FIG. 4 are the “MSK” modulation(curve a) and “MSK16” modulation (curve b) for an input signalcomprising the data −1, 1, 1, −1, 1, 1, −1, 1, −1, 1. The differencesbetween these two curves appear only at the changes of frequency.

The phase variations of this input signal are illustrated in FIG. 5:curves a and b corresponding respectively to the “MSK” and “MSK16”modulations. The phases vary by ±π/2 over T in both cases, the phase ofcurve b) first lagging slightly behind that of curve a) and subsequentlycatching it up.

The frequency spectrum of an “MSK16” modulation at 400 baud exhibits, asillustrated in FIG. 6, curve b a faster decrease than the spectrum of an“MSK” modulation at 400 baud also, illustrated by curve a. At 1000 Hz,one clearly obtains a sidelobe situated at around −50 dB of the centrallobe and the sidelobes are much less marked.

This staged change of frequency furthermore exhibits the advantage ofconsuming less energy at high frequency than during instantaneouschanges of frequency, as in the case of “MSK” modulation, this energygain then being in part carried over to the central lobe of the spectrumwhich is flatter in the case of “MSK16” modulation than in that of “MSK”modulation and thus gives it a slightly wider useful band and thereforea signal/noise ratio higher by around 2 dB, for identical conditions.

As has been seen, as far as reception is concerned, conventional phaseor frequency demodulation corresponding to “MSK” modulation may beperformed.

As represented in FIG. 7, the method according to the invention isimplemented by a modulator comprising a device 1 for shaping themodulation as 16 stages, which is linked to a device 2 able to generatethe instantaneous frequency using, for example, a “DDS” function, theacronym standing for “Direct Digital Synthesis”, this device itselfbeing linked to a power amplifier 3. When the carrier frequency lies ina frequency range other than the HF range, a frequency transpositioncircuit may possibly be added between the device 2 and the amplifier 3.

The device 1 for shaping the modulation as 16 frequency stages comprisesmeans 10 for programming the 16 stages, included for example in amicroprocessor, these means 10 being linked to means 20 for shaping themodulation as a function of the data to be transmitted and of thefrequency stages such as programmed.

These means 20 for shaping the modulation may be included in themicroprocessor.

These means 20 for shaping the modulation preferably comprise an “FPGA”,the acronym standing for “Field Programmable Gate Array”, linked to themicroprocessor. The “FPGA” comprises, on the one hand and traditionally,means 21 for temporally adapting the data to be transmitted and, on theother hand, means 22 for providing the “DDS” type device with theinstructions for generating the frequencies corresponding to the datamodulated according to an “MSK16” modulation.

Such a modulator which makes it possible to obtain a frequency spectrumexhibiting the same advantages as that obtained by a “GMSK”-typemodulator, is however of simpler design insofar as it does not use anyGaussian filter.

Moreover, by eliminating the Gaussian filter, it eliminates thecorresponding part of the analog processing (filtering) of the data,thereby simplifying overall the management and the reliability of themodulator.

Furthermore, it exhibits no inter-symbol crosstalk.

1. A method of transmitting digital data exhibiting a rate T by means ofa frequency modulator able to modulate as a function of the data, acentral carrier frequency f0 at a first frequency value f0+¼T and/or asecond frequency value f0−¼T, method including the step of: modulatingthe central carrier frequency f0 from one of the frequency values to theother during a time interval T, via successive frequency stages.
 2. Themethod as claimed in claim 1, wherein the number of frequency stages forgoing from f0−¼T to f0+¼T is the same as for going from f0+¼T to f0−¼T.3. The method as claimed in claim 2, wherein the frequency stages forgoing from f0−¼T to f0+¼T are the same in absolute value as thefrequency stages for going from f0+¼T to f0−¼T.
 4. The method as claimedin, claim 1 wherein the number of frequency stages for going from one ofthe frequency values to the other is equal to
 16. 5. The method asclaimed in, claim 1 wherein the digital data are transmitted between areference station and a mobile of a satellite-based positioning system.6. A device able to implement a frequency modulation at a rate Taccording to two predetermined frequencies, comprising: a device forshaping the said modulation as several frequency stages during a timeinterval T.
 7. The device as claimed in claim 6, comprising amicroprocessor able to program the frequency stages.
 8. The device asclaimed in claim 7, furthermore comprising, linked to themicroprocessor, a device of “FPGA” type able to shape the modulation asa function of data to be transmitted and of the programmed stages.
 9. Afrequency modulator, comprising a device able to shape a frequencymodulation according to claim 6 and, linked to it, a device forgenerating instantaneous frequencies using a “DDS” function.
 10. Themethod as claimed in claim 2, wherein the number of frequency stages forgoing from one of the frequency values to the other is equal to
 16. 11.The method as claimed in claim 3, wherein the number of frequency stagesfor going from one of the frequency values to the other is equal to 16.12. The method as claimed in claim 2, wherein the digital data aretransmitted between a reference station and a mobile of asatellite-based positioning system.
 13. The method as claimed in claim3, wherein the digital data are transmitted between a reference stationand a mobile of a satellite-based positioning system.
 14. The method asclaimed in claim 4, wherein the digital data are transmitted between areference station and a mobile of a satellite-based positioning system.